Half bridge converters are known in the state of the art; in offline applications two topologies of half bridge converters are used: the asymmetrical half bridge converter, shown in FIG. 1, and the LLC resonant half bridge converter (so called because the tank circuit comprises the leakage inductance and the magnetization inductance of the transformer and a capacitor) shown in FIG. 2. The power switches Q1 and Q2 of the above mentioned converters are driven in push-pull with a small delay time inserted between the switching of each one. This delay time assures that the two switches of the half bridge will not be turned on simultaneously and allows soft-switching at turn-on, that is a switching modality according to which the MOS transistors of the power switches of the half bridge goes from the cut-off condition to the saturation condition without dissipating power during the transition.
In the asymmetrical half bridge converter the switches are driven with a complementary duty-cycle. The switching frequency of the half bridge is fixed and the regulation of the output voltage against variations of the input voltage and the load is achieved by modulating the duty cycle. Since the operating frequency of the half bridge is higher than the resonance frequency of the circuit comprising the capacitor C and the inductance of the primary of the transformer, the waveforms of the currents are piecewise linear.
The LLC resonant half bridge converter comprises power switches which are driven by means of a duty cycle fixed at 50% and the regulation of the output voltage against variations of the input voltage and the load is achieved by modulating the switching frequency of the half bridge. In this case the operating frequency of the half bridge moves about the resonance frequencies of the LLC tank and the current waveforms are piecewise sinusoidal.
In general, overload and the short-circuit are the heaviest operating conditions for switching converters. In the first case a current higher than the maximum specified is requested from the converter (for example for an anomaly of operation of the load) while in the second case a very low impedance path is present at the output terminals (for example because of a load failure), which forces an output current much larger than the maximum specified. In these cases it is necessary to protect the converter by limiting the output average current at lower values so as to prevent power elements from overheating and not to endanger the reliability of the power supply unit.
In all converters this is done by inserting sensing means of the currents flowing through one or more power elements and by activating the protection circuits when the read current level is higher than a preset value.
In PWM (Pulse Width Modulation) controlled converters, like the asymmetrical half bridge converter, since energy is controlled by the duty cycle of the switches, the protection circuit can act by reducing the duty cycle. In the LLC converter energy is controlled by the operating frequency and the protection circuit acts by increasing the operating frequency.
The protection circuits must allow a maximum power flow higher than the value corresponding to the maximum load, to account for converter's circuits; therefore the energy level is still large enough to damage some of the power components of the converter. Then, it is necessary to shut down the converter and let it restart after a certain time period; in this way, if the overload/short-circuit lasts long, the converter will operate intermittently, thus reducing the average value of the energy.
The temporary shutdown and the consequent onset of intermittent operation must be delayed for a certain time period. This is required because the protection circuits must assure immunity to temporary phenomena such as, for example, a temporary current absorption higher than the maximum specified.
It is therefore necessary that the converter be able to operate under overload or short-circuit conditions for a limited time period, referred to as “tolerance time”. This must be short enough to prevent dangerous thermal stress but not too much, to prevent malfunctions at start-up or in presence of pulsed loads.
To solve this problem one can follow two ways. In those applications wherein the control device of the converter is supplied by an independent supply line that is not conditioned by the operation of the same converter, user-programmable timers force converter's shutdown when the converter operates under anomalous conditions for more than a preset time and allow its restart after another preset time period.
In applications wherein the supply line of the control device is derived from the same converter, delayed shutdown is often automatically obtained because the voltage produced by the self-supply system is not sufficient to keep the voltage above a certain value under which the control device (and, as a consequence, the entire converter) shuts down. In this case the control device shuts down after a certain time related to the discharge time of the reservoir capacitor of the self-supply system. Once shut down, its dissipation is very low and the current sourced by the start-up circuit of the converter charges the capacitor, increasing the supply voltage until the control device and the converter turn on.
In half bridge converters, in addition to the aforementioned issues, there is a problem due to the presence of the blocking capacitor.
In the asymmetrical half-bridge, since under overload conditions the duty cycle is 50%, the capacitor, which is quite big, will be charged at about half the input voltage. At converter's start-up after the temporary shutdown as above mentioned, the capacitor is still charged and causes considerable unbalance of the voltages at the terminals of the transformer. This determines high currents until the capacitor's voltage approaches the balance value; in this way it is possible that the transformer saturates temporarily, with current levels difficult to control.
In the case of LLC converter the typical values of the capacitor are smaller; however the voltage values which can be observed under short-circuit condition are higher; the situation is not much different from the case of the asymmetrical half-bridge, although the presence of the leakage inductance reduces the saturation problems of the transformer.
Two ways are followed to solve the above mentioned problems. One way is that of placing discharge means of the capacitor, such as a resistance in parallel to the capacitor; this has the disadvantage of being dissipative, thus hurting converter's efficiency. Another way is that of adding a current sensing circuit and further protection circuits to limit the current peaks which occur at restart. This solution is very expensive.